Studies at Mount Hood, east of Portland, Ore., showed that most of the magma stored underneath is relatively cold until just before eruption. (Photo: Shutterstock)

When the U.S. Geological Survey monitors volcanoes for signs of imminent eruption, it uses various instruments on the ground, in the air, and affixed to satellites in space to search for tell-tale heat signatures. Geologists also look for other clues, like strange gases streaming out of the rocky crevices or unusual seismic activity. But there may be another way, according to new research published in Nature on Sunday.

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By studying historical eruptions of Mount Hood, east of Portland, Ore., scientists discovered that magma stored beneath active volcanoes is actually relatively cold most of the time. Only at the very last minute, just before an eruption, does liquid hot magma push itself to the surface. In other words, there's a special condition that must be activated — the rise of fresh hot magma — in order for Mount Hood to erupt.

"If you can see a body of magma that has a high amount of liquid, perhaps this magma is getting ready to erupt or at least has some potential to erupt," study co-author Adam Kent, a geologist at Oregon State University, told LifeScience. "It wouldn't be a slam-dunk guarantee." Of course, not all volcanoes are Mount Hood. But study lead Kari Cooper tells LiveScience that "our expectation is that there [are] a lot of volcanoes that behave this way."

It's still unclear how many do. And that includes the recently devastating Mount Kelud volcano in Indonesia on the island of Java. Its fatal eruption this month buried surrounding villages in a thick coat of ash and forced the evacuation of thousands of people. As for Mount Hood, its last eruption was sometime in the first half of the 19th century.

Cooper and Kent studied this eruption and one other dating back 1,500 years ago using crystallized magma, which they dated using radioactive decay of uranium inside. According to NewScientist, they discovered that these crystals were 21,000 years old. They knew they had something unusual when they found more strontium in the core of the crystal than on the edges. Here's the weird part: strontium melts at 1,382 degrees Fahrenheit (750 degrees celsius), and if the magma were as hot as most people thought, it would've oozed out to the edge of the crystal.

The scientists ran some calculations and realized that the magma under Mount Hood could only ever have been hotter than 1,382 degrees Fahrenheit for between 140 to 2,800 years. At most that's 12 percent of the crystal's lifetime. But they say it's probably less than 1 percent.

Here's what else they know. If the magma underneath a volcano like Mount Hood is more than 50 percent crystalline, it's not going to erupt. But if liquid hot magma bubbles up and melts more than 50 percent of the crystals, the volcano could blow. They were basically able to measure this with their historical Mount Hood data. "We can see chemical traces of new magma reacting [with the old], and the time to eruption was only days to weeks, maybe months," Cooper told LiveScience.